Liquid crystal display device with arrangement of common electrode portion and image signal electrode

ABSTRACT

A liquid crystal display device has a plurality of pixel elements, the optical transmissivity of which is varied by suitable electrical signals. The liquid crystal display device has electrodes which apply electric fields to a liquid crystal layer, the electric fields having components in a direction generally parallel to the liquid crystal layer. Each pixel element has at least one pixel electrode which extends in a common direction as a signal electrode and common electrodes which extend over several pixel elements. The common electrodes may be on the same side of the liquid crystal layer as the pixel and signal electrodes, or they may be on opposite sides. Each pixel may have two pixel electrodes with the signal electrode disposed therebetween, and there are then a pair of common electrodes with the pixel electrodes therebetween. The common electrodes may be common to adjacent pixel elements. The pixel electrodes and the common electrodes may be separated by an insulating film.

This application is a continuation of application Ser. No. 08/744,451,now U.S. Pat. No. 5,737,051 filed Nov. 6, 1996, which is a continuationof application Ser. No. 08/123,472, filed Sep. 20, 1993, now U.S. Pat.No. 5,598,285.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device havingat least one, preferably a plurality, of pixel elements.

2. Description of the Prior Art

In standard liquid crystal display device, the pixel element is a liquidcrystal layer (normally common to the pixel elements) extending in aplane, and there is at least one polarizing means parallel to the planeof the liquid crystal layer. By applying electrical signals to theliquid crystal layer using suitable electrodes, it is possible to varythe angle of polarization of polarized light passing through the liquidcrystal layer. Thus, by changing those electrical signals, it ispossible to vary the optical transmissivity of a liquid crystal displaydevice by varying the change in polarization relative to the at leastone polarizing means. Normally, in such a liquid crystal display device,the polarizing means is formed by two polarizing plates, one on eachside of the liquid crystal layer, but is also possible to provide anarrangement with a single polarizing plate on one side of the liquidcrystal layer, and a reflective element on the other side of the liquidcrystal element.

In standard liquid crystal display devices, electrical fields aregenerated by the electrodes perpendicular to the plane of the liquidcrystal layer. Therefore, if the change in the liquid crystal layer dueto the electric fields is to be visible, the extent of those electrodesneeds to be large, and therefore it is necessary to use transparentelectrodes. Furthermore, at least two layers are normally needed betweenthe transparent electrodes on each side of the liquid crystal layer andthe liquid crystal layer itself. One layer forms an orientation layerfor the liquid crystal layer, but a further insulating layer is thenneeded between the orientation layer and the transparent electrode.

In International Patent Application No. PCT WO91/10936, a liquid crystaldisplay device was disclosed in which electrical signals were applied tothe liquid crystal layer so as to generate electric fields havingcomponents in a direction parallel to the plane of the liquid crystallayer. Such parallel field components cause reorientation of themolecules of the liquid crystal layer, thereby varying the opticaltransmissivity of the liquid crystal display device.

In PCT WO91/10936, it was proposed that the electrodes for applying suchfield were, for each pixel element, in the form of combs, the teeth ofthe comb formed by one electrode extending into the spaces between theteeth of the comb formed by the other electrode. The teeth of eachelectrode were electrically connected in common, and a voltage wasapplied between the electrodes.

JP-B-63-21907(1988) also disclosed a liquid crystal display device inwhich electrical signals were applied to the liquid crystal layer so asto generate electric fields having components in a direction parallel tothe plane of the liquid crystal layer. As in PCT WO91/10936, theelectrodes for applying such fields were, for each pixel element, in theform of combs. Use of comb-shaped electrodes was also disclosed in U.S.Pat. No. 4,345,249.

In each of these known arrangements, each pixel element thus has firstand second electrodes of comb shape, with the teeth of one combextending between the teeth of the other comb. Voltages are then appliedto the electrodes by a suitable control circuit. It is important to notethat the teeth of the comb-shaped electrodes are not electricallyindependent, so that size of the pixel is determined by the size of thecomb-shaped electrode.

The principles of operation of such devices, with comb shapedelectrodes, is also discussed in an article entitled "Field Effects InNematic Liquid Crystals Obtained With Interdigital Electrodes by R. A.Soref in the Journal of Applied Physics, pages 5466 to 5468, vol. 45,no. 12 (December 1974), and in article entitled "InterdigitalTwisted-Nematic-Displays" by R. A. Soref published in the Proceedings ofthe IEEE, pages 1710 to 1711 (December 1974).

SUMMARY OF THE PRESENT INVENTION

In the standard liquid crystal display devices discussed above, it isnecessary to use transparent electrodes, which are formed on facingsurfaces of two substrates. However in order to form such transparentelectrodes, it is necessary to use a vacuum manufacturing operation,such as sputtering, and thus the cost of manufacture of such standardliquid crystal display devices is high. Furthermore, it has been foundthat such transparent electrodes have vertical geometricalirregularities, of the order of several tens of nanameters, this preventprecise manufacture of active devices, such as thin film transistorsneeded to control the signals to the electrodes. Also, it has been foundthat parts of such transparent electrodes may come detached, to causepoint or line defects. Thus, it has proved difficult to manufacture bothreliably and cheaply liquid crystal devices.

Such conventional liquid crystal display devices also have disadvantagesin terms of picture quality. The problem of vertical geometricalirregularities in the transparent electrodes has been mentioned above,but similar irregularities around the controlling transistors may resultin orientation failure domains being formed, requiring light shieldingfilm to cover such transistor devices, using the light utilisationefficiency of the liquid crystal device. Also, such conventional liquidcrystal display devices have disadvantage that their significant changein brightness when the visual angle is changed, and reversion of somegradation levels can occur in a half-term display, at some view angles.

Although the use of comb shaped electrodes, such as previouslydiscussed, prove the need for transparent electrodes, further problemshave been found. While the use of such comb-shaped electrodes offerstheoretical advantages, those are limited by practical considerationwhich have to be taken into account when the comb-shaped electrodes areused. If the teeth of such comb-shaped electrodes have a width of 1 to 2micrometers, satisfactory practical operation can be achieved. However,It is extremely difficult to form such fine teeth over a large substratewithout defects. Thus, in practice, the aperture factor of the liquidcrystal display device is reduced, because of the need to providerelatively wide electrode teeth. There is thus a trade-off betweenaperture factor and production yield, which is undesirable.

Therefore, the present invention seeks to provide a liquid crystaldisplay device which is more suitable for mass production than the knownliquid crystal display devices discussed above. The present inventionhas several aspects.

In the liquid crystal display device according to the present inventionthere are features which are common to all the aspects. The device has aliquid crystal layer, and at least one polarising means, which isnormally a pair of polarising plates on opposite sides of the liquidcrystal layer. The device has at least one, normally a plurality, ofpixel elements and there are electrodes which receive electrical signalsfor controlling the optical transmissivity of light through the device.As in e.g. JP-B-63-21907 (1988) discussed above, the electrical signalsare applied such that electrical fields are generated in the liquidcrystal layer with components parallel to the plane of the liquidcrystal layer. The various aspects of the present invention, which willbe discussed below, then relate to the electrode arrangement of a pixelelement(s) and also to the materials and optical arrangements of thematerials of the liquid crystal display device.

In a first aspect of the present invention, each pixel element has apixel electrode extending in a first direction within the pixel, andthere are also signal wiring electrodes extending in the same directionover several of the pixel elements. There are also common electrodesextending in that first direction over more than one of the pixelelements.

There may be a pair of pixel electrodes for each pixel element, with thesignal wiring electrodes then extending between a pair of pixelelectrodes at each pixel element. There is then a pair of commonelectrodes, with the pair of pixel electrodes being therebetween, sothat electrical fields are generated in opposite directions for eachpixel element.

Preferably, all the electrodes are on the same side of the liquidcrystal layer. Arrangements are also possible, however, in which thecommon electrodes are on the opposite side of the liquid crystal layerof the other electrodes. In either case, if there is insulating materialbetween the common electrode and the pixel electrode for each pixelelement, a capacitive device may be formed therebetween.

In practice, it is possible for the common electrodes to be in commonfor two adjacent pixel elements, by interacting with pixel electrodes onopposite sides of each common electrode.

In a second aspect of the present invention, each pixel can beconsidered to have a elongate transistor element extending in a firstdirection, that elongate transistor element having at least one elongateelectrode. There is also at least one elongate common electrodeextending in the same direction as the elongate transistor element. Inthe second aspect, an insulating film separates the at least oneelongate electrode of the elongate transistor element and the at leastone common electrode.

In a third aspect of the present invention, each pixel has again anelongate transistor element extending in one direction, and at least oneelongate common electrode extending in the same direction. The elongatetransistor element has at least one elongate electrode, and there is aninsulating film extending over and being in direct contact with that atleast one elongate electrode. That insulating film is also in directcontact with the liquid crystal layer. Preferably, that insulating filmis an organic polymer.

In the fourth aspect of the present invention, each pixel can beconsidered to have a transistor element with a pair of first electrodes(pixel electrodes), a signal electrode between the first electrodes, anda gate electrode. The first and second electrodes extend in a commondirection, as do a pair of common electrodes. A transistor element isthen (in plan) between the pair of common electrodes. As has previouslybeen mentioned, the common electrodes and the transistor element may beon the same side of the liquid crystal layer, or may be on oppositesides.

In the fifth aspect one pixel element, each one of said pairs of commonelectrodes thereof forms a corresponding one of said pair of commonelectrodes pixel elements adjacent to said any one pixel element.

The five aspects of the present invention discussed above all relate tothe electrode arrangement of the liquid crystal display device. Theaspects of the invention to be discussed below relate to the opticalarrangement and materials of the liquid crystal display device.

In a sixth aspect of the present invention the angles between componentsof electric fields in a direction parallel to the plane of said liquidcrystal layer and the direction of orientation of molecules at oppositesurfaces of the liquid crystal layer are the same, and the product ofthe thickness of the liquid crystal layer and the refractive indexanisotropy of the liquid crystal layer is between 0.21 μm and 0.36 μm.

In a seventh aspect of the present invention, the absolute value of thedifference between the angles between components of electric fields in adirection parallel, to the plane of said liquid crystal layer and thedirection of orientation of molecules at opposite surfaces of the liquidcrystal layer is not less than 80° and not greater than 100°, andproduct of the thickness of the liquid crystal layer and the refractiveindex anisotropy of the liquid crystal layer is between 0.4 μm and 0.6μm.

In a eighth aspect of the present invention, dielectric constantanisotropy of the liquid crystal layer is positive, and the absolutevalue of the angle between components of electric fields in a directionparallel to the plane of said liquid crystal layer and the direction oforientation of molecules at the surface of the liquid crystal layer isless than 90° but not less than 45°.

In a ninth aspect of the present invention, dielectric constantanisotropy of the liquid crystal layer is negative and the absolutevalue of the angle between components of electric fields in a directionparallel to the plane of said liquid crystal layer and the direction oforientation of molecules at the surface of the liquid crystal layer isgreater than 0° but not greater than 45°.

In a tenth aspect of the present invention dielectric constantanisotropy of the liquid crystal layer is positive, and the value of thedifference between: i) the angle between components of electric fieldsin a direction parallel to the plane of said liquid crystal layer andthe direction of orientation of molecules at the surface of the liquidcrystal layer; and ii) the angle of the polarization axis of said atleast one polarizing plate and said components of electric fields in adirection parallel to the plane of said liquid crystal layer, is 3° to15°.

In a eleventh aspect of the present invention dielectric constantanisotropy of the liquid crystal layer is negative, and the value of thedifference between i) the angle of the polarization axis of said atleast one polarizing plate and said components of electric fields in adirection parallel to the plane of said liquid crystal layer, and ii)the angle between components of electric fields in a direction parallelto the plane of said liquid crystal layer and the direction oforientation of molecules at the surface of the liquid crystal layer is30° to 15°.

In an twelfth aspect of the present invention direction of orientationof molecules of said liquid crystal layer at a surface of said liquidcrystal layer parallel to the plane of said liquid crystal layer andsaid surface is not more than 4°.

Although various aspects of the present invention have been discussedabove, a liquid crystal display device embodying the present inventionmay incorporate combinations of such aspects. Depending on the resultingcombination, the present invention provides advantages in themanufacture and/or operation of a liquid crystal display device, andthese advantages will be discussed in more detail later.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described in detail, byway of example, with reference to the accompanying drawings, in which:

FIGS. 1(a) to 1(d) are schematic diagrams illustrating the behaviour ofliquid crystal molecules in a liquid crystal display device according tothe present invention;

FIGS. 2(a and 2(b) are plan and sectional views respectively of anembodiment, and show the structure of a thin film transistor which maybe used in the present invention;

FIGS. 3(a) and 3(b) are graphs illustrating electro-opticalcharacteristics (visual angle dependance), FIG. 3(a) corresponding tothe present invention and FIG. 3(b) corresponding to a comparativeexample.

FIG. 4 shows an embodiment of the present invention in which the pixelelectrode (source electrode), the common electrode, the scanningelectrode, and the signal electrode (drain electrode) are all arrangedon only one of the substrates of the device;

FIGS. 5(a) and 5(b) are plan and sectional views of an embodiment of thepresent invention in which the pixel electrode (source electrode) andthe signal electrode (drain electrode) are arranged at the center of thepixel to divide the pixel into two parts;

FIG. 6 is a diagram illustrating the angles formed by the long axisorientation direction of the liquid crystal molecules at the interfacethe polarizing axis of the polarizing plate, and the phase advance axisof the phase difference plate, with respect to the direction of theelectric field;

FIG. 7 is a graph illustrating the electro-optical characteristics ofvarious embodiments with different orientation directions of the longaxis of the liquid crystal molecules at the interface when thedielectric constant anisotropy is positive;

FIG. 8 illustrates schematically a liquid crystal display drivingcircuit system which may be used in the present invention;

FIG. 9 illustrates an example of the present invention applied to aliquid crystal display transmission type optical system;

FIG. 10 illustrates an example of the present invention applied to aliquid crystal display reflection type optical system;

FIG. 11 is a graph illustrating the electro-optical characteristics ofvarious embodiments with different orientation directions of the longaxis of the liquid crystal molecules at the interface when thedielectric constant anisotropy is negative;

FIGS. 12(a) and 12(b) are graphs illustrating the electro-opticalcharacteristics of various embodiments with different thicknesses d ofthe liquid crystal layer when the dielectric constant anisotropy isnegative;

FIGS. 13(a) and 13(b) are graphs illustrating the electro-opticalcharacteristics of embodiments in which polarizing plates are arrangedso that the dark state can be obtained by applying a small non-zeroelectric field;

FIG. 14 is a graph illustrating the characteristics of a normally opentype device and the characteristics when its residual phase differenceat the interface is compensated;

FIG. 15(a) is a plan view and FIGS. 15(b) and 15(c) a sectional views ofa fourth embodiment in which the capacitive device is formed between acommon electrode and the pixel electrode on the respective facinginterface of the substrates;

FIG. 16 is a schematic plan view of a pixel when no electric field isapplied, for a fifth embodiment of the present invention in which thepixel electrode and the common electrode are arranged at differentlayers separated by an insulating layer;

FIG. 17 is a schematic cross sectional view of a pixel when no electricfield is applied in the fifth embodiment in which the pixel electrodeand the common electrode are arranged at different layers separated byan insulating layer;

FIG. 18 is a schematic plan view of a pixel when no electric field isapplied in a sixth embodiment of the present invention in which thepixel electrode and the common electrode are arranged at differentlayers separated by an insulating layer, the pixel electrode is a closedloop and the common electrode is cruciform.

FIG. 19 is a schematic plan view of a pixel when no electric field isapplied in a seventh embodiment of the present invention in which thepixel electrode and the common electrode are arranged at differentlayers separated by an insulating layer, the pixel electrode has theshape of the letter I, and the common electrode is a closed loop;

FIG. 20 is a schematic cross sectional view of part of a pixel when noelectric field is applied in an eighth embodiment of the presentinvention in which the pixel electrode and the common electrode arearranged at different layers separated by an insulating layer, and thereis an insulating layer between the scanning electrode and the commonelectrode;

FIG. 21 is a schematic cross sectional view of part of a pixel when noelectric field is applied in a ninth embodiment of the present inventionin which the pixel electrode and the common electrode are arranged atdifferent layers separated by an insulating layer, and the commonelectrode is formed on the protection insulating film; and

FIG. 22 is a schematic cross sectional view of part of a pixel when noelectric field is applied in a tenth embodiment of the present inventionin which the pixel electrode and the common electrode are arranged atdifferent layers separated by an insulating layer, and both the scanningelectrode and the common electrode are made from aluminum coated withself-oxidized film.

DETAILED DESCRIPTION

Before describing embodiments of the present invention, the generalprinciples underlying the present invention will be explained.

First, suppose that for the angle of a polarized light transmission axisof the polarized plate with respect to the direction of the electricfield is φ_(P), the angle of the longitudinal axis (optical axis) ofliquid crystal molecules near the interface with respect to the electricfield is φ_(LC), and the angle of the phase advance axis of the phasedifference plate inserted between the pair of polarizing plates withrespect to the electric field is φ_(R), as shown in FIG. 6. Becausethere are normally pairs of polarizing plates and liquid crystalinterfaces, i.e. upper and lower plates and interface, these arerepresented as φ_(P1), φ_(P2), φ_(LC1), and φ_(LC2). FIG. 6 correspondsto a top view of FIG. 1, to be described later.

Next, the operation of embodiments of the present invention will bedescribed with reference to FIGS. 1(a) to 1(d).

FIGS. 1(a) and 1(b) are side cross sections showing the operation of aliquid crystal display device according to the present invention. FIGS.1(c) and 1(d) are front views of the liquid crystal display device. InFIGS. 1(a) to 1(d), the thin film transistor device is omitted. In thisinvention, stripes of electrodes are used to form a plurality of pixels,but the FIGS. 1(a) to 1(d) show the structure of only one pixel (onecell). The side cross section of the cell when no voltage is applied isshown in FIG. 1(a), and the top view is shown in FIG. 1(c).

Linear electrodes 1, 2, are formed on the inner sides of pairedtransparent substrates 3, over which an orientation control film 4 isapplied and subjected to an orientation processing. A liquid crystal isheld between the paired substrates 3. Bar shaped liquid crystalmolecules 5 of the liquid crystal are oriented so that they are at aslight angle to the longitudinal direction of the electrodes 1, 2, i.e.45 degrees≦φ_(LC) <90 degrees, when no electric field is applied. Thisdiscussion assumes that the orientation directions of the liquid crystalmolecules on the upper and lower interfaces are parallel to each other,i.e. φ_(LC1) =φ_(LC2). It is also assumed that the dielectric constantanisotropy of the liquid crystal is positive.

When an electric field 7 is applied, the liquid crystal molecules 5change their directions and are aligned with the direction of theelectric field 7, as shown in FIGS. 1(b) and 1(d). By arranging thepolarizing plates 6 at a specified angle 9, it is possible to change thelight transmission factor when the electric field is applied. Thus, itis possible to provide a display having a contrast without usingtransparent electrodes.

In FIG. 1(b), the angle between the substrate surface and the directionof the electric field looks large and it seems not to be parallel to thesubstrates. In reality this is the result of magnifying the thicknessdirection in FIG. 1(b) and the angle is actually less than 20 degrees.

In the following description, the electric fields whose inclination isless than 20 degrees are generally referred to as lateral electricfields. FIGS. 1(a) and 1(b) show an arrangement in which the electrodes1, 2, are provided separately on the upper and lower substratesrespectively. However, it is possible to arrange them on one substrateand produce the same effect. Because the wiring pattern is very fine andmay therefore deform due to heat and/or external forces, arranging theelectrodes on a single substrate is preferable because it permits moreprecise alignment.

Although the dielectric constant anisotropy of the liquid crystal isassumed to be positive, it may be negative. In that case, the initialorientation of the liquid crystal is set so that the liquid crystalmolecules have a slight angle |φ_(LC) |(i.e. 0 degree<|φ_(LC) |≦45degrees) with respect to a direction perpendicular to the longitudinaldirection of the electrodes 1, 2, (the direction of the electric field7).

Advantages which may thus be achieved with the present invention willnow be explained.

(1) The first advantage is that enhanced contrast may be achievedwithout using transparent electrodes.

There are two types of structure for producing contrast. The oneutilizes a mode in which the orientations of the liquid crystalmolecules 5 on the upper and lower substrates 3 are almost parallel toeach other (since color interface by birefringent phase difference isused, this mode is called a birefringent mode). The other structure hasa mode in which the orientations of the liquid crystal molecules 5 onthe upper and lower substrates 3 cross each other, twisting themolecular orientation in the cell (since light spiraling produced byrotation of the polarization plane in the liquid crystal compositionlayer is used, this mode is called a light spiraling mode).

In the birefringent mode, when a voltage is applied, the molecular longaxis 8 (optical axis) changes its direction in a plane almost parallelto the substrate interface, changing its angle with respect to the axisof a polarized plate (not shown in FIGS. 1(a) and 1(b)) which is set ata specified angle. This results in a change in the light transmissionfactor.

In the light spiraling mode, the application of a voltage similarlychanges only the direction of the molecular long axis in the same plane.This mode, however, utilizes a change in the light spiraling as thespiral is unraveled.

Next, a structure for making the display colorless and increasing thecontrast ratio explained below hereinafter for two cases: one using thebirefringent mode and the other using the light spiraling mode.

II. Displaying in birefringent mode: generally, when a uniaxialbirefringent medium is inserted between two orthogonal polarizingplates, the light transmission factor, T/T₀, is expressed as follows.

    T/T.sub.0 =sin.sup.2 (2χ.sub.eff)·sin.sup.2 (πd.sub.eff ·Δn/λ)                              Equation (1)

In Equation 1, χ_(eff) is the effective direction of light axis ofliquid crystal composition layer (an angle between the light axis andthe polarized light transmission axis), d_(eff) is the effectivethickness of the liquid crystal composition layer having birefringence,Δn is the refractive index anisotropy, and λ is the wave length of thelight. In an actual cell the liquid crystal molecules are fixed at theinterface and not all the liquid crystal molecules in the cell areparallel and uniformly oriented in one direction when an electric fieldis applied. Instead they are significantly deformed particularly nearthe interface. It is therefore convenient to assume an apparent uniformstate as the average of these states. In Equation 1, an effective valueis used for the light axis direction of the liquid crystal compositionlayer.

To obtain a normally closed characteristic in which the display appearsdark when a low voltage V_(L) is applied and bright when a high voltageV_(H) is applied, the polarizing plates should be arranged so that thelight transmission axis (or absorption axis) of one of the polarizingplates is almost parallel to the orientation direction of the liquidcrystal molecules (rubbing axis), i.e. φ_(P2) ≈φ_(LC1) ≈φ_(LC2). Thelight transmission axis of the other light polarizing plate isperpendicular to the first axis, i.e. φ_(P2) =φ_(P1) +90°. When noelectric field is applied, since χ_(eff) in Equation 1 is zero, thelight transmission factor T/T₀ is also zero. On the other hand, when anelectric field is applied, the value of χ_(eff) increases in dependenceon the intensity of the electric field and becomes maximum at 45degrees. Then, if the light wavelength is assumed to be 0.555 μm, inorder to make the light colorless and the light transmission factormaximum, the effective value of d_(eff) ·Δn should be set to half of thewavelength, i.e. 0.28 μm. Actually, however, there is a margin in thevalue, and values between 0.21 μm and 0.36 μm and suitable, with thevalues between 0.24 μm and 0.33 μm being preferred.

On the other hand, in order to obtain a normally open characteristics inwhich the display appears bright when a low voltage V_(L) is applied anddark when a high voltage V_(H) is applied, the polarizing plates need bearranged so that χ_(eff) in Equation 1 is almost 45° when no electricfield is applied or an electric field of low intensity is applied. Whenan electric field is applied, the value χ_(eff) decreases in dependenceon the field intensity as opposed to the case of the normally closedcharacteristic. However, because there is a residual phase difference ofthe liquid crystal molecules fixed near the interface even when theχ_(eff) is minimum (i.e. zero), a significant amount of light will leakunder this condition.

In an experiment conducted by the inventors of the present invention, inwhich the value of d·Δn was set between 0.27 and 0.37 and an effectivevoltage of 3 to 10 volts was applied, the residual phase difference onthe interface was 0.02 to 0.06 μm. Hence, by inserting a phasedifference plate having a birefringence phase difference of about 0.02to 0.06 μm (this phase difference is represented as R_(f)) to compensatefor the interface residual phase difference, the dark state becomesdarker giving a high contrast ratio. The angle φ_(R) of the phaseadvance axis of the phase difference plate is set parallel to theeffective light axis χ_(eff) of the liquid crystal composition layerwhen the voltage is applied.

To make the dark state be as black as possible, the angle of the phaseadvance axis should be aligned precisely with the residual phasedifference that occurred when a voltage for displaying a dark state isapplied. Therefore, to make the dark state compatible with the increasedlevel of transmission factor and lightness of the bright state, thefollowing relationship must be fulfilled.

    0.21 μm<(d·Δn-R.sub.f)<0.36 μm        Equation 2

Or more preferably,

    0.23 μm<(d·Δn-R.sub.f)<0.33 μm        Equation 3

II. Displaying in light spiraling mode: In a conventional twistednematic (TN) system, when the value of d·Δn is set to around 0.50 μm, afirst minimum condition, a high transmission factor and colorless lightmay be obtained. It has been found preferable to set the value in arange from 0.40 to 0.60 μm. The polarizing plates are arranged such thatthe transmission axis (or absorbing axis) of one of the polarizingplates is set almost parallel to the orientation direction (rubbingaxis) of the liquid crystal molecules on the interface, i.e. φ_(LC1)≈φ_(LC2). For realize a normally closed type device, the transmissionaxis of the other polarizing plate is set parallel to the orientationdirection of the liquid crystal molecules, and for a normally open type,the transmission axis of the polarizing plate is set perpendicular tothe orientation direction.

To eliminate light spiraling it is necessary to set the orientationdirection of the liquid crystal molecules near the upper and the lowersubstrate interfaces so that they are almost parallel to each other. Ifa 90° TN mode is assumed, the liquid crystal molecules on one of thesubstrates must be turned nearly 90°. However, in displaying in thebirefringence mode, the liquid crystal molecules need only be turnedabout 45°. Furthermore, the birefringence mode has a lower thresholdvalue.

(2) The second advantage is that the visual angle characteristics may beimproved.

In the display mode, the long axes of the liquid crystal molecules arealmost parallel to the substrate at all times and do not becomeperpendicular to the substrate, so that there is only a small change inbrightness when the visual angle is changed. This display mode gives adark state, not by making the birefringence phase difference almost zeroby applying voltage as in the case of a conventional display device, butby changing the angle between the long axes of liquid crystal moleculesand the axis (absorbing or transmission axis) of the polarizing plate.Thus, the display mode of the present invention differs fundamentallyfrom that of the conventional device.

In a conventional TN type of liquid crystal display deice in which thelong axes of the liquid crystal molecules are perpendicular to thesubstrate interface, the birefringence phase difference of zero isobtained only in a visual direction perpendicular to the front or thesubstrate interface. Any inclination from this direction results in abirefringence phase difference, which means leakage of light in the caseof the normally open type, causing reduction of contrast and reversal oftone levels.

(3) The third advantage is that there is improved freedom in theselection of the materials of the orientation film and/or the liquidcrystal, and margin for the related process may thus be increased.

Since the liquid crystal molecules do not become erect, an orientationfilm for providing a large inclination angle (the angle between the longaxis of the liquid crystal molecule and the interface of the substrate),which was used in a conventional device, is no longer necessary. In aconventional liquid crystal display device, when the inclination anglebecomes insufficient, two states with different inclination directionsand domains bordering the two states occur, giving a possibility to be apoor display. Instead of having such an inclination angle, the displaysystem of the present invention may have the long axis direction of theliquid crystal molecule (rubbing direction) on the substrate interfaceto be set in a specified direction different from 0° or 90° with respectto the electric field direction.

For example, when the dielectric constant anisotropy of the liquidcrystal is negative, the angle between the electric field direction andthe long axis direction of the liquid crystal molecule on the substrateinterface φ_(LC) (φ_(LC) >0) need exceed 0° normally more than 0.5°,preferably more than 2°. If the angle is to be set exactly 0° degree,two kinds of deformations with different directions, and domains of twodifferent states and their bordering are generated, and a possibility ofdeterioration in display quality occurs. If the angle is set to morethan 0.5°, the apparent long axis direction of the liquid crystalmolecule (φ_(LC) (V)) increases uniformly with increasing intensity ofelectric field, and there is no possibility of the long axis beinginclined in the reverse direction, i.e. φ_(LC) (V)<0.

With this system, since no domains occur even if the angle (inclinationangle) between the interface and the liquid crystal molecule is small,it is possible to set the angle to have a small value. The smaller theinclination angle, the greater the process margin for rubbing will beimproving, the uniformity of the liquid crystal molecule orientation.Hence, when the present process in which an electric field is suppliedin parallel to the interface combines with a low inclination technique,the orientation of the liquid crystal molecule becomes more uniform, anddisplay variations can be reduced much better than in a conventionalsystem even if there are variations of the same magnitude in themanufacturing process.

Generally, there are fewer kinds of orientation films that produce highinclination angle than those giving small inclination angle. The presentsystem increases freedom in the selection of the orientation filmmaterial. For example, when an organic polymer is used for theflattening film over the color filters and for the protective film overthe thin film transistors and is directly subjected to the surfaceorientation processing such as rubbing, the organic film can be usedwith ease as the orientation film simultaneously because there is noneed to provide an inclination angle. Hence, it becomes possible tosimplify the process and to decrease the cost. In order to eliminatedisplay irregularities due to variations in the manufacturing process,the inclination angle preferably is set below 4° more preferably below2°.

Furthermore, freedom in the selection of the liquid crystal material canbe increased, as will be explained below.

In the present invention, the pixel electrodes and the common electrodesmay have a structure in which an electric field generally parallel tothe interface of the substrate is applied to the liquid crystalcomposition layer. The distance between the electrodes can be chosen tobe longer than the distance between the mutually facing transparentelectrodes of the conventional vertical electric field active matrixtype liquid crystal display device. The equivalent cross section area ofthe electrode can be made smaller than that of the conventionalarrangement. Hence, the electric resistance between the paired pixelelectrodes of the present invention can be significantly larger thanthat of the mutually facing transparent electrodes of the conventionalactive matrix liquid crystal display device.

Furthermore, the electrostatic capacitance between the pixel electrodeand the common electrode of the present invention can be connected inparallel with capacitive devices, and the capacitive device having ahigh electric resistance can be achieved. Therefore, the electric chargeaccumulated in the pixel electrode can be held with ease, and sufficientholding characteristics can be achieved even if the area of thecapacitive device is decreased. This area reduces the aperture factorand therefore needs to be small, if possible.

Conventionally the liquid crystal composition has an extremely highspecific resistance, for instance 10¹² Ωcm. However, in accordance withthe present invention,composition having a lower sped crystalcomposition having a lower specific resistance than the conventionalone, without causing any problems. That means not only increased freedomfor selection of the liquid crystal material, but also increases of themargin for the processing. In other words, a defect in display qualityrarely occurs even if the liquid crystal is contaminated during theprocessing. Thus, the a margin for the previously described variation atthe interface with the orientation film increases, and defects caused atthe interfaces are rare. Consequently, processes such as inspection andageing can be significantly simplified, and the present invention cancontribute significantly to a lowering in the cost of thin filmtransistor type liquid crystal display devices.

Because the present invention permits the pixel electrode to have a moresimple shape than the known comb shaped electrode, the efficiency ofutilization of the light is increased. It is does not necessary tosacrifice some of the aperture factor, as in conventional methods forobtaining a capacitive device which can accumulate sufficient amount ofelectric charge. Replacement of the insulator for protecting the thinfilm transistors with an organic composition enables the dielectricconstant to be lower than that when an inorganic composition is used,making it possible to suppress the electric field component generated inthe vicinity of the pixel electrode in a vertical direction to theinterface of the substrate smaller than the electric field in a lateraldirection. This enables the liquid crystal to operate in a wider region.It also contributes to an enhancement of brightness. When the commonelectrode is used in common as the electrode for the adjacent pixelelectrodes, it operates in the same way as the common electrode in theconventional active matrix type liquid crystal display device, but itsstructure can be simplified as compared with more than conventionalarrangements, and the aperture factor can be further increased.

As there is increased freedom in the selection of materials for theliquid crystal, the orientation film, and the insulator, it becomespossible to select insulating materials of the capacitive devices sothat the product of their specific resistance and dielectric constant islarger than that of the material of the liquid crystal. Then, onevertical scanning period in the driving signal output from the scanningwiring driving means can be set shorter than the time constant expressedby a product of the specific resistance and the dielectric constant ofthe insulator of the capacitive devices. Hence, voltage variation at thepixel electrode can be reduced to sufficiently small value.

(4) The fourth advantage is that a simple thin film transistor structurehaving a high aperture factor can be achieved, permitting enhancement ofbrightness.

Consider the structure of a pixel including a thin film transistor, whencomb shaped electrodes are used as disclosed in (JP-B-63-21907 (1988).There is then the problem that the aperture factor decreasessignificantly and brightness is lowered. For mass productivity thenecessary width of one tooth of the comb shaped electrode is about 8 μm,with a minimum of at least 4 μm. Hence, it is impossible to form a pixelof 0.3×0.1 mm² for a diagonal length 9.4 inches (23.9 cm) color VGAclass with a structure having a total of 17 teeth as shown in FIG. 7 ofJP-B-63-21907 (1988).

The present invention permits a sufficient aperture factor to bemaintained, without losing the described advantages (1) and (2)discussed above. Instead of structures such as comb shaped electrodeswhich inevitably reduce the aperture factor, the more simple structureof the electrode permits a practical high aperture factor to beachieved.

The first aspect of the invention discussed above relates to structuresin which the common electrodes are formed on the mutually facinginterfaces of substrates, or the pixel electrodes are formed on a samelayer. In (JP-B-63-21907 (1988)), the directions of the signal wiringand the common electrode cross over at the right angles to each other inorder to form the comb shaped electrodes. That is, the signal wiringextends in a first direction (Y direction), and the common electrodeextends in a direction perpendicular to the first direction (Xdirection). On the other hand, the present invention permits theavoidance of a complex structure such as comb shaped electrodes byhaving the signal wiring, the pixel electrode, and the common electrodeall extending in a common direction. In order to reduce the thresholdvoltage of the liquid crystal and to shorten the response time, it ispreferable to make the interval between the pixel electrode and thecommon electrode narrow, locating the pixel electrode and the signalwiring electrode between a pair of common electrodes is also effectiveand it is not necessary to use a complex structure such as a comb shapedelectrode.

The second aspect of the present invention also permits the structure tobe simplified and the aperture factor increased by providing the pixelelectrode and the common electrode in different layers separated fromeach other by an insulating layer. This aspect of the present inventiondiffers substantially from JP-B-63-21907 (1988) in that the pixelelectrode and the common electrode are provided in separated layers. Oneadvantage of this is that the region for the additional capacitivedevice which has been reduced by use of the lateral electric fieldsystem can be further reduced. Thus, overlapping of the pixel electrodeand the common electrode separated by an insulating film becomespossible because they are in separate layers, and a load capacitance canbe formed in the region of overlap. The overlapping parts can be used asa part of the wiring for the common electrodes. Hence, it is notnecessary to sacrifice a part of the display in order to form acapacitive device. Accordingly, the aperture factor for the pixel can befurther increased.

Depending on a design of each pixel, a plurality of capacitive devicescan be formed. Hence, the voltage holding characteristics may besignificantly improved, and deterioration of the display quality rarelyoccurs even if severe contamination of the liquid crystal and loweringof off-resistance of the thin film transistor are generated.Furthermore, the insulating film formed between the pixel electrode andthe common electrode can be commonly used in common as a gate insulatingfilm of the scanning wiring (gate line) in the same layer. Therefore,there is no need to form a further film, and a process step forproviding separate layers is not necessary.

There are other advantages of the separate layers formed by insertion ofthe insulating film between the pixel electrode and the common electrodefor example the probability of short-circuit failures between the pixeland common electrodes can be reduced significantly due to the existenceof the insulating film therebetween, and accordingly, the probability ofpixel defects can be reduced.

The common electrode and/or the pixel electrode, preferably have shapesmaking a pattern which makes the aperture factor as large as possible.The pixel electrode or the common electrode has any of a flat shapeselected from the group of shapes of a ring, a cross, a letter T, aletter Π, a letter I, and a ladder. By suitably combining the selectedshapes the aperture factor can be increased significantly, as comparedwith the case using comb shaped electrodes.

Because the common electrode and the pixel electrode are in separatelayers with an insulating film therebetween, it becomes possible toprovide electrodes having shapes which overlap each other. Hence, thepresent invention permits an increase in the aperture factor. When thecommon electrode is composed of a metallic electrode of which surface iscoated with self-oxidized film or self-nitrized film, short-circuitfailure between the common electrode and the pixel electrode can beprevented even if the two electrodes mutually overlap, so that the highaperture factor and the prevention of the pixel defects are compatible.

Embodiments of the present invention will now be described in detail.

Embodiment 1

In the first embodiment shown in FIGS. 2(a) and 2(b) two glasssubstrates (not shown in FIG. 2(a) or 2(b) but as shown in FIGS. 1(a) to1(d)) were used which were polished on the surfaces and 1.1 mm thick.Between these substrates was interposed a nematic liquid crystal whichhad a positive dielectric constant anisotropy Δ.di-elect cons. of 4.5and birefringence Δn of 0.072 (589 nm, 20° C.). A polyimide orientationcontrol film applied over the substrate surface was subjected to rubbingprocessing to produce a pretilt angle of 3.5 degrees. The rubbingdirections of the upper and the lower interfaces were almost paralleland at an angle of 85 degrees (φ_(LC1) =φ_(LC2) =85°) with respect tothe direction of the applied electric field.

A gap d was formed by dispersing spherical polymer beads between thesubstrates so that the gap became 4.5 μm when the liquid crystal wassealed. Hence, Δn·d was 0.324 μm. The resulting structure was clamped bytwo polarizing plates (not shown) (manufactured by e.g. Nitto Denko,with reference G1220DU). One of the polarizing plates had its polarizedlight transmission axis set in almost parallel to the rubbing direction,i.e. φ_(P1) =85°. The polarized light transmission axis of the otherpolarizing plate was set perpendicular to the former, i.e. φ_(P2) =-5°.As a result, a normally closed characteristics was obtained.

The structure of the thin film transistor and various electrodes for onepixel element are as shown in FIG. 2(a) and FIG. 2(b), such that thethin film transistor device (hatched portion in FIG. 2(a) has a pixelelectrode (source electrode) 1, a signal electrode (drain electrode) 12,and a scanning electrode (gate electrode) 10. The pixel electrode 1extends in a first direction (the vertical direction in FIG. 2), thesignal electrode 12 and the common electrodes 2 extend in the firstdirection, and extend so as to cross over a plurality of pixels (thepixels being arranged vertically in FIG. 2), and the thin filmtransistor device is located between the common electrodes 2.

Signal waves having information are supplied to the signal electrode 12,and scanning waves are supplied to the scanning electrode 10synchronously. A channel layer 16 composed of amorphous silicon (a--Si)and the thin film transistor composed from an insulating protective film15 of silicon nitride (SiN) are arranged between adjacent commonelectrodes. Information signals are transmitted from the signalelectrode 12 to the pixel electrode 1 through the thin film transistor,and a voltage is generated between the common electrode 2 and the liquidcrystal 50.

In the present embodiment, the common electrodes 2 are arranged at thefacing interface side of the substrate and is enlarged in the thicknessdirection in the illustration in FIG. 2(b). Therefore, although theelectric field direction 7 shown in FIG. 2(b) appears to be inclinedrelative to the horizontal, the thickness of the liquid crystal layer 5is actually about 6 μm as compared with the width of 48 μm, so that theinclination is very small and the supplied electric field direction isalmost parallel to the interface of the substrate.

A capacitive device 4 was formed in a structure in which the protrudedpixel electrode 1 and the scanning wiring 10 held a gate insulating film13 therebetween as shown in FIG. 1(c). The electrostatic capacitance ofthe capacitive device 11 was about 21 fF. Each of the lines of thescanning wiring 10 and the signal wiring 11 were connected to a scanningwiring driving LSI and a signal wiring driving LSI respectively.

Electric charge accumulates in the pixel electrode 1 to about 24 fF,which is a capacitance of parallel connection of the electrostaticcapacitance between the pixel electrode 1 and the common electrode 2 andthat of the capacitive device 11. Therefore, even if the specificresistance of the liquid crystal 50 is 5×10¹⁰ Ωcm, the voltage variationat the pixel electrode 1 could be suppressed, and deterioration of thedisplay quality was prevented.

In this embodiment the number of the pixels were 40 (×3)×30, and pixelpitch was 80 μm in a lateral direction (i.e. between the commonelectrodes) and 240 μm in a vertical direction (i.e. between thescanning electrodes). A high aperture factor of e.g. 50% was obtained,with the scanning electrode 10 being of 12 μm wide and the intervalbetween the adjacent scanning electrodes being 68 μm. Three stripeshaped color filters 17 being respective red (R), green (G) and blue (B)were provided on the substrate facing the substrate supporting the thinfilm transistors. On the color filter.

A transparent resin 14 was laminated on the color filters 17 for surfaceflattening. The material of the transparent resin 14 was preferably,epoxy resin. An orientation control film 4 e.g. of polyimide group, wasapplied to the transparent resin. A driving circuit was connected to thepanel.

The structure of the driving circuit system in the present embodiment isshown in FIG. 8. A signal electrode 23 and a common electrode 31 extendto the end of the display portion. FIGS. 9 and 10 show the structure ofthe optical systems needed, FIG. 9 for being for a transmission typedevice and FIG. 10 being for a reflection type device with a reflector30.

As the present embodiment does not need any transparent electrodes, themanufacturing process becomes simple, the production yield increases,and the manufacturing cost can be significantly reduced. In particular,there is no need for extremely expensive facilities having vacuumfurnaces for forming the transparent electrodes and there may thus be asignificant reduction in investment in the manufacturing facilities,permitting and or accompanying cost reduction.

The electro-optical characteristics showing the relationship between theeffective voltage applied to the pixels and the brightness in thepresent embodiment is shown in FIG. 3(a). The contrast ratios exceeded150 when driven by voltages of e.g. 7 V. The difference between thecharacterising when the visual angle was changed laterally or verticallywere significantly smaller than in conventional system (to be discussedin comparative example 1), and the display characteristics was notchanged significantly even if the visual angle was changed. Orientationcharacter of the liquid crystal was preferable, and domains oforientation defects are not generated. The aperture factor maintained asufficiently high value, e.g. 50% by simplifying the structure of thethin film transistor and the electrodes, and a bright display wasachieved. The average transmission factor for the whole panel was 8.4%.It should, be noted that the term brightness is defined as thebrightness of transmission when the two polarizing plates are arrangedin parallel.

The material of the liquid crystal 50 used in the first embodiment had adielectric constant of 6.7 and a specific resistance of 5×10¹⁰ Ωcm, andsilicon nitride was used for the insulator of the capacitive device 11had a dielectric constant of 6.7 and a specific resistance of 5×10¹⁶Ωcm. That means that the specific resistances of both the liquid crystalcomposition and the insulator of the capacitive device 11 were over 10¹⁰Ωcm, and the product of the dielectric constant and the specificresistance of the silicon nitride, about 3×10⁴ seconds, was larger thanthe product of the dielectric constant and the specific resistance ofthe liquid crystal composition about 0.03 seconds. One vertical scanningperiod for the driving signal output from the scanning wiring drivingLSI was about 16.6 ms with an ordinary liquid display device, and thevalue satisfied the condition that the scanning period should be farless than about 3×10⁴ seconds. Therefore, it was possible to derive thetime constant for accumulated charge leaking from the pixel electrode 1.This facilitates the suppression of voltage variations at the pixelelectrode 1, and consequently a satisfactory display quality can beobtained. The value of 5×10¹⁰ Ωcm for the specific resistance of theliquid crystal is lower than that for the liquid crystal used for theconventional vertical electric field thin film transistor liquid displaydevice, which is about 10¹² Ωcm. However, defects in the display qualitywere not generated.

COMPARISON EXAMPLE

The comparison example referred to above was based on a conventionaltwisted nematic (TN) type of liquid crystal display device. Since thisexample had a transparent electrode, the structure was complex and themanufacturing process was long compared with the first embodiment. Thenematic liquid crystal used in the comparison example had a dielectricconstant anisotropy Δ.di-elect cons. of positive 4.5 and a birefringenceΔn of 0.072 (589 nm, 20° C.), the same as those of the Embodiment. Thegap was set to 7.3 μm and the twist angle to 90 degrees. Thus, Δn·d is0.526 μm.

The electro-optical characteristic of this comparison example is shownin FIG. 3(b). The characteristic curves change greatly as the visualangle changes. Near a stepped portion adjacent to the thin-filmtransistor, an orientation failure domain occurs where the liquidcrystal molecules are oriented in a direction different from that of thesurrounding portion.

Embodiment 2

In the second embodiment, the scanning electrode which had been arrangedon the substrate facing the substrate supporting the pixel in the firstembodiment was formed on the same substrate as the pixel electrode. Therest of the structure of the second embodiment is generally the same asthat of the first embodiment and corresponding parts are indicated bythe same reference numerals. The cross section of the structure of thethin film transistor and the electrodes in the second embodiment areshown in FIG. 4. The pixel electrode 1, the signal electrode 12, and thescanning electrode 10 were all made from aluminum, and were formedsimultaneously by deposited and etched. There is no conductive materialon the other substrate. Hence, in this structure, even if the conductivematerial is contaminated during the manufacturing process, there is nopossibility of upper and lower electrodes touching each other, anddefects due to the upper and lower electrodes touching is eliminated.There is no special restriction on the material for the electrodes, butit should normally be a metal having low electric resistance, andchromium, copper, etc are thus suitable.

Generally, the precision of alignment of photomasks is significantlyhigher than that for two facing glass substrates. Therefore, variationsin the alignment of the electrodes can be suppressed when all of thefour electrodes are formed on only one of the substrates, as in thesecond embodiment because alignment of the electrodes duringmanufacturing can be only photomasks. Therefore, the present embodimentis suitable for forming more precise patterns in comparison with thecase when the scanning electrode is formed on the other substrate.

A bright display having the same wide visual angle as the firstembodiment was obtained.

Embodiment 3

The structure of the third embodiment is generally the same as the firstembodiment 1 except as will be described below. Components whichcorrespond to components of the first embodiment are indicated by thesame reference numerals.

The structure of the thin film transistor and the various electrodes ofthe third embodiment is shown in FIGS. 5(a) and 5(b). The signalelectrode 12 was arranged between a pair of pixel electrodes 1 and apair of common electrodes 2 were arranged outside the above electrodes.A signal wave having information is applied to the signal electrode 12,and a scanning wave is applied to the scanning electrode 10synchronously with the signal wave. A thin film transistor comprisingamorphous silicon (a--Si) 16 and an insulating-protecting film 15 ofsilicon nitride (SiN) is arranged substantially centrally between a pairof common electrodes. The same information signals are transmitted fromthe signal electrode 12 to each of two pixel electrodes 1 through twothin film transistors, and the same voltage signals are applied to theliquid crystal, and each of two common electrodes at both sides have thesame potential. With this arrangement, the a distance between theelectrodes can be decreased by almost a half without making thestructure of the thin film transistor and the electrodes complex. Itthus becomes possible to apply a high electric field with the samevoltage, and decrease the driving voltage. Hence, a high response can beachieved.

This embodiment permits the same brightness and wide visual angle to beachieved as in the first embodiment.

Embodiment 4

The structure of the fourth embodiment is generally the same as thefirst embodiment except as will be described below. Components of thefourth embodiment which correspond to the first embodiment are indicatedby the same reference numerals.

FIG. 15(a) is a partial plan view of an active matrix type liquidcrystal display device being the fourth embodiment. FIG. 15(b) is across sectional view taken on line A-A' in FIG. 15(a), and FIG. 15(c) isa cross sectional view taken on line B-B' in FIG. 15(a). The capacitivedevice 11 which had a structure in which the gate insulating filmcomposed from silicon nitride 13 was located between the pixel electrode1 and the scanning wiring 10 in the first embodiment 1 was changed to astructure in which the liquid crystal composition layer 50 extendedbetween parts of the pixel electrode 1 and the common electrode 2 whichfaced each other, as shown in FIG. 15(c).

The fourth embodiment enables an electrostatic capacitance of thecapacitive device 11 to be connected in parallel to an electrostaticcapacitance between the pixel electrode 1 and the common electrode 2.Hence, any voltage variation at the signal wiring 10 does not affect thepixel electrode 1. Therefore, the voltage variation at the pixelelectrode 1 could be reduced, reducing variations in the display.

There was no deterioration in display quality with the active matrixtype liquid crystal display device of this fourth the presentembodiment, and the same advantages as the first embodiment wereobtained.

Embodiment 5

The structure of each of the fifth to tenth embodiments are generallythe same as the first embodiment, except as will be described below.Corresponding parts are indicated by the same reference numerals.

FIGS. 16 and 17 respectively show a plan view and a cross sectional viewof an unit pixel of the fifth embodiment, in which the pixel electrode 1and the common electrode 2, are located on the same side of the liquidcrystal material and are separated by an insulating layer. A scanningelectrode 10 and a common electrode 2 of chromium were formed on a glasssubstrate, and a gate insulating film 13 of silicon nitride (SiN) wasformed so as to cover the above electrodes. An amorphous film (a--Si) 16was formed on a part of the scanning electrode 10 with the gateinsulating film 13 therebetween as an active layer of the transistor.

A signal electrode 12 and a pixel electrode 1 of molybdenum were formedto overlap on a part of the pattern of the a--Si film 16, and aprotection and insulating film 15 of SiN film was formed so as to coverthe resulting structure. When the thin film transistor was operated byapplying a voltage to the scanning electrode 13 of the thin filmtransistor, a voltage is applied to the pixel electrode 1. When anelectric field is induced between the pixel electrode 1 and the commonelectrode 2 the liquid crystal molecules change their orientation in thedirection of the electric field and the light transmission changes.

In the fifth embodiment, the common electrode 2 was formed on the samelayer as the scanning electrode 10, and the pixel electrode 1 and thesignal electrode 12 were separated from the common electrode 2 by thegate insulating film 13. The device of the fifth embodiment thus differsfrom that of JP-B-63-21907 (1988), in that conventional comb shapedelectrodes are not used, and the pixel electrode 1 and the commonelectrode 2 overlap with the gate insulating film 20 therebetween. Byseparating the pixel electrode 1 and the signal electrode 12 from thecommon electrode 2 by insulation, design freedom for the plan pattern ofthe pixel electrodes 1 and the common electrodes 2 is increased, and itbecomes possible to increase the pixel aperture factor.

The overlapping parts of the pixel electrode 1 and the common electrode2 operate as an additional capacitance which is connected in parallel tothe liquid crystal capacitance, and accordingly, it becomes possible toincrease the holding ability of the liquid crystal charged voltage. Thisadvantage cannot be achieved by conventional comb shaped electrode, andthe advantages are achieved only by separating the pixel electrode 1 andthe signal electrode 12 from the common electrode by insulation manner.As FIG. 16 reveals, it is not necessary to form a capacitive device bysacrificing a part of the display region, as in the case when the pixelelectrode and the common electrode are formed on the same substrate, andall that is needed is to make an overlap in a part of the wiring forleading out the common electrode outside the display region.

As described above, since the design freedom for the plan patternincreases by forming the pixel electrode 1 and the signal electrode 12in a separate layer from the layer having the common electrode 2,various types and shapes for the electrodes can be adoptednotwithstanding the present embodiment.

Embodiment 6

FIG. 18 shows an plan view of a unit pixel in the sixth embodiment ofthe present invention in which the pixel electrode 1 and the commonelectrode 2 are located in different layers separated by an insulatinglayer. The cross sectional structure of the sixth embodiment is the sameas that of the fifth embodiment (FIG. 17).

In the sixth embodiment, the common electrode 2 is cruciform and thepixel electrode 1 is forms a closed loop. The pixel electrode 1 and thecommon electrode 2 overlap at parts C1, C2, C3 and C4 in FIG. 18, so asto form additional capacitances. In the sixth embodiment, the distancebetween the common electrode 2 and the scanning electrode 10 can be madewide, and thus failures due to short circuits between the commonelectrode 2 and the scanning electrode 10 can be prevented. Since thepixel electrode 1 is in the form of a closed loop, normal operation canbe maintained as power is supplied to all parts of the pixel electrodeeven if a breaking occurs at an arbitrary portion of the pixelelectrode, unless a break occurs at two or more parts. That means, thatthe structure of the sixth embodiment has a redundancy for breaking ofthe pixel electrode 1 and the production yield can therefore beincreased. On account of the redundancy, it becomes possible to reducethe wiring gap of the closed loop electrode 1, to make that electrode 1close to the wiring part of the scanning electrode 10 when they arearranged in different layers, so increasing the aperture factor.

Embodiment 7

FIG. 19 shows a plan view of a unit pixel in a seventh embodiment of thepresent invention in which the pixel electrode 1 and the commonelectrode 2 are located in different layers separated by an insulatinglayer. The cross sectional structure of the seventh embodiment is thesame as that of the fifth embodiment (FIG. 17).

In the sixth embodiment, the pixel electrode 1 is in the shape of aletter I, and the common electrode 2 is in the form of a closed loop. Inthe seventh embodiment, the aperture factor can be improved as in thesixth embodiment, and the additional capacitance can be increasedbecause the overlapping of the pixel electrode 1 and the commonelectrode 2 can be increased.

Embodiment 8

FIG. 20 shows a plan view of a unit pixel in an eighth embodiment of thepresent invention in which the pixel electrode 1 and the commonelectrode 2 are located in different layers separated by an insulatinglayer.

In the eighth embodiment, the common electrode 2 is mounted on thesubstrate 3 and separated from the scanning electrode 10 by a beddinglayer insulating film 23. Thus, the common electrode 2 is located on thesubstrate 3, and is in a different layer separate from the layersforming the scanning electrode 10, the pixel electrode 1, and the signalelectrode 12. Therefore, in accordance with the eighth embodiment, itbecomes possible for the common electrode 2 to extend not only parallelto the scanning electrode 10 but also a perpendicular to the scanningelectrode 10 to form a network structure. Therefore, the resistance ofthe common electrode 2 can be decreased, and accordingly, reductions ofthe wave distortion in the common voltage and prevention of smeargeneration can be achieved.

Embodiment 9

FIG. 21 shows a plan view of unit pixel in a ninth embodiment of thepresent invention in which the pixel electrode 1 and the commonelectrode 2 are located in different layers separated by an insulatinglayer.

In the ninth embodiment, the common electrode 2 is mounted on aprotecting-insulating film 15. In accordance with the presentembodiment, the common electrode 2 is located in a different layerseparate from the layers forming all the scanning electrode 10, thepixel electrode 1, and the signal electrode 12, as the eighthembodiment. Therefore, it is possible for the common electrode 2 toextend not only parallel to the scanning electrode 10 but alsoperpendicular direction to the scanning electrode 10 to form a networkstructure. Accordingly, the resistance of the common electrode 2 can bereduced, wave distortion in the common voltage can be decreased, andsmear generation can be prevented.

Embodiment 10

FIG. 22 shows a plan view of a unit pixel in a tenth embodiment of thepresent invention in which the pixel electrode 1 and the commonelectrode 2 are located in different layers separated by an insulatinglayer.

In the tenth embodiment, the scanning electrode 10 and the commonelectrode 2 are made from aluminum (Al), and surfaces of thoseelectrodes 2,10 are covered by alumina (AL₂ O₃) 32 which is aself-oxidized film of aluminum. The adoption of such a structure havingdouble insulating layers decrease, failure of the insulation between thecommon electrode 2, the signal electrode 12, and the pixel electrode 1,and accordingly, pixel failure can be decreased.

Embodiment 11

The structure of the eleventh embodiment is generally the same as thefirst embodiment except as will be described below. Corresponding partsare indicated by the same reference numerals.

In the eleventh embodiment a flattening film 14 (FIG. 2(b)) made from atransparent polymer was laminated on the color filters 27 as an organicinsulating layer, and the surface of the film was subjected to a directrubbing treatment without forming other films such as an orientationcontrol film on the flattening film 14. Epoxy resin was used for thematerial of the transparent film. The epoxy resin has two functionsnamely of flattening and orientation control of the liquid crystalmolecules. The liquid crystal composition layer contacted directly withthe epoxy resin, and the inclination angle of the interface was 0.5degrees.

This structure eliminates the need to provide orientation film, andmakes the manufacturing easier and shorter.

Generally, in the conventional twisted nematic (TN) type, a variety ofcharacteristics are required for the orientation film, and it wasnecessary to satisfy all of those above requirements. Therefore, thematerial for the orientation film was limited to a small variety ofmaterials such as polyimide etc. The inclination angle is the mostimportant characteristic. However, as explained previously, the presentinvention does not require a large inclination angle and thus the rangeof material selection is significantly increased.

Measurement of electro-optical characteristics of the eleventhembodiment revealed that, as in the first embodiment, the change in thecharacteristic curves was extremely small when the visual angle waschanged laterally and vertically, and the display characteristics hardlychanged. Although the inclination angle was as small as 0.5 degrees, theliquid crystal orientation was satisfactory, and no orientation failuredomain was generated.

Embodiment 12

In the twelfth embodiment, the transparent resin forming the flatteningfilm 14 in the eleventh embodiment was changed from epoxy resin topolyimide resin. The surface of the polyimide resin was directly rubbedso that it had both the function of flattening and the function oforientation control of the liquid crystal molecules. The inclinationangle on the interface was 2 degrees. In comparison with otherembodiments, the display characteristics hardly changed. The liquidcrystal orientation was satisfactory and no orientation failure domainwas generated.

Embodiment 13

The structure of the thirteenth embodiment was the same as the firstembodiment 1 except as will be described below. Corresponding parts areindicated by the same numerals.

The protection film 15 (FIG. 2(b)) of silicon nitride for protecting thethin film transistor was in the first embodiment replaced with anorganic insulating layer made from epoxy resin. The surface of the epoxyresin was directly treated by rubbing so that it functions both as aflattening film and an orientation control film for the liquid crystalmolecules. The inclination angle was 0.5 degrees.

Measurement of the electro-optical characteristics of the thirteenthembodiment revealed that the display characteristics were hardly changedin comparison with the first embodiment 1 . Although the inclinationangle was as small as 0.5 degrees, in the eleventh embodiment, theliquid crystal orientation was satisfactory, and no orientation failuredomain was generated.

Embodiment 14

In the fourteenth embodiment, the epoxy resin used as the protectionfilm in the thirteenth embodiment was replaced with an organicinsulating layer made from polyimide resin.

Measurement of the electro-optical characteristics in the fourteenthembodiment revealed that the display characteristics were hardly changedin comparison with the first embodiment 1. The inclination slightlyincreased angle to 2.0 degrees in comparison with the thirteenthembodiment. The liquid crystal orientation was satisfactory, and noorientation failure domain was generated.

Embodiments 15-19

The structure of the fifteenth to nineteenth embodiments were the sameas the fourteenth embodiment except as will be described below.

In the fifteenth embodiment, the directions of the long axes of theliquid crystal molecules on the upper and the lower interfaces (therubbing direction) were almost in parallel to each other and set at 89.5degrees (φ_(LC1) =φ_(LC2) =89.5°) with respect to the applied electricfield. The polarized light transmission axis of one of the polarizingplates was set almost in parallel to the rubbing direction (φ_(P1)=89.5°) and the polarized light transmission axis of the otherpolarizing plate was set perpendicular to the first axis (φ_(P2)=-0.5°).

Similarly, in the sixteenth embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =88°,and φ_(P2) =-2.0°.

Similarly, in the seventeenth embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =75°,and φ_(P2) =-25°.

Similarly, in the eighteenth embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =45°,and φ_(P2) =-45°.

Similarly, in the nineteenth embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =30°,and φ_(P2) =-60°.

The results of the measurement of the electro-optical characteristicsfor these embodiments are shown in a single diagram of FIG. 7. In theseembodiments, the brightness was expressed by a normalized value suchthat the maximum brightness in a range of applied voltage from zero voltto 10 volts (effective value V_(rms)) was 100% and the minimumbrightness was 0%. There was a tendency for the characteristics curvesof threshold to become steep as the angle φ_(LC) was increased. In orderto provide a large voltage margin for half-tone, the angle φ_(LC) mustbe reduced. However, there was a tendency that when the φ_(LC) wassmaller than 45 degrees, brightness decreased. The optimum value of theangle φ_(LC) depends on the number of the half-tone levels to bedisplayed, the requirement for brightness, driving voltage, and whetheror not the common electrode has a voltage applied thereto. A designercan control the threshold characteristics in a wide range by suitablyselecting the angle φ_(LC). When considering brightness, the angle ispreferably φ_(LC) at least 45 degrees. An angle between 60 degrees and89.5 degrees is more preferable.

Measurement of the visual angle characteristics revealed that, in eachcase, the characteristics curve changed very slightly when the visualangle was changed laterally and vertically, and the displaycharacteristics were hardly changed, as in the first embodiment 1.

The liquid crystal orientation was satisfactory, and no orientationfailure domain was generated.

Embodiments 20-23

The greatest difference between the foregoing embodiments previouslydescribed and the twentieth to twenth-third embodiments was that thedielectric constant anisotropy of the liquid crystal composition layerwas set to be negative and the rubbing direction was changedaccordingly. A nematic liquid crystal (e.g. that known as ZLI-2806 ofMerck Co.) with Δ.di-elect cons. of -4.8, and Δn of 0.0437 (589 nm, 20°C.) was used. In the twentieth to twenth-third embodiments, thedirections of the long axes of liquid crystal molecules on the upper andthe lower interfaces (the rubbing directions, (φ_(LC1), φ_(LC2)) wereapproximately parallel (φ_(LC1) =φ_(LC2)) to each other and set at anangle, φ_(LC1), exceeding zero degree and less than 45 degrees withrespect to the applied electric field. The polarized light transmissionaxis of one of the polarizing plates was set approximately parallel tothe rubbing direction (φ_(P1)) and the polarized light transmission axisof the other polarizing plate was set perpendicular to the first axis(φ_(P2)).

Thus, in the twentieth embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =1.5°, andφ_(P2) =-88.5°.

In the twenty-first embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =15°, andφ_(P2) =75°.

In the twenty-second embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =30°, andφ_(P2) =60°.

In the twenty-third embodiment, φ_(LC1) =φ_(LC2) =φ_(P1) =45°, andφ_(P2) =-45°.

The gap d was set to be 6.3 μm with the liquid crystal under sealedconditions and the Δn·d was set to be 0.275 μm. Other conditions such asthe structure of the thin film transistor, and the electrodes were thesame as the third embodiment.

The results of the measurement of the electro-optical characteristics inthese embodiments are shown in a single diagram of FIG. 11. Unlike tothe case when the dielectric constant anisotropy was positive, there wasa tendency for the characteristic curves of the threshold to becomesteep as the angle φ_(LC) was decreased. In order to provide a largevoltage margin for half-tone, the angle φ_(LC) must be increased.However, there was a tendency that when the φ_(LC) was larger than 45degrees, brightness decreased. As in the case when the dielectricconstant anisotropy was positive, the optimum value of the angle φ_(LC)depends on the number of the half-tone levels to be displayed, therequirement for brightness, driving voltage, and whether or not thecommon electrode has a voltage applied. A designer can control thethreshold characteristics in a wide range by suitably selecting theangle φ_(LC). When considering the brightness, the angle φ_(LC) isalmost 45 degrees.

Measurement of the visual angle characteristics revealed that, in eachcase, the characteristics curve changed very slightly when the visualangle was changed laterally and vertically, and the displaycharacteristics were hardly changed, as in the first embodiment. Noreversions of levels in the half-tone display (8 steps) were observed ina range ±50 degrees both laterally and vertically. The liquid crystalorientation was satisfactory, and no orientation failure domain wasgenerated.

Embodiments 24 to 26

In these embodiments, the directions of the long axes of liquid crystalmolecules and the arrangement of the polarizing plates were set to bethe same as the twenty-first embodiment which produced the bestcharacteristics among the embodiments 20 to 23 (φ_(LC1) =φ_(LC2) =φ_(P1)=15°, and φ_(P2) =-75°). Only the product, d·Δn, of the thickness of theliquid crystal composition layer d and refractive index anisotropy Δnwas changed. In the twenty-fourth, twenty-fifth and twenty-sixthembodiments the thickness d of the liquid crystal composition layer wereset to be 4.0, 4.9, and 7.2 μm, respectively. Thus, the product, d·Δn,was 0.1748, 0.2141, and 0.3146 μm, respectively. In these embodiments,the refractive index anisotropy Δn was a constant and only the thicknessof the liquid crystal composition layer d was changed. However, as wellas the other type of the liquid crystal display (such as 90 degreestwisted nematic type), the same result for the optimum brightness can beobtained even if the refractive index anisotropy Δn is changed.Moreover, the same result can be obtained even if the liquid crystalcomposition layer has a positive dielectric constant anisotropy.

The results of the measurement in these embodiments are shown in FIG.12. The abscissa in FIG. 12(a) indicates applied voltages, and theabscissa in FIG. 12(b) indicates d·Δn with the applied voltage being aconstant 7 Volts. As FIG. 12(b) reveals, the brightness depends stronglyon d·Δn and an optimum value exists. In order to maintain a brightnessof at least 30%, which is preferable for practical use, the value ofd·Δn should be between 0.21 and 0.36 μm, and further if the brightnessexceeding 50% is desired, the value of d·Δn between 0.23 and 0.33 μmmust be selected. In consideration of sealing time for the liquidcrystal, thickness control of the liquid crystal composition layer etc,and mass-productivity, the value for d must be at least 5.0 μm, and Δnof almost 0.08 is preferable.

Embodiments 27 to 29

As the results of the twenty-fourth to twenty-sixth embodiments reveal,the optimum value of d·Δn is between 0.21 and 0.36 μm, preferablybetween 0.23 and 0.33 μm. Since the thickness of the liquid crystalcomposition layer which is preferable for mass-production is at least5.0 μm, the value of the refractive index anisotropy Δn must be almost0.072, preferably almost 0.066. However, the kinds of liquid crystalcompounds having such an extremely low value for Δn are very limited,and it is very difficult for them to be compatible with othercharacteristics required for practical use.

Therefore, a new method was considered in which d and Δn of the liquidcrystal composition layer are set rather higher than the optimum values,and an optically anisotropic medium having a lower value for phasedifference Rf than the value for d·Δn of the liquid crystal compositionlayer is provided. This compensates for the departure from the optimumvalue by the phase difference of with the liquid crystal compositionlayer. As the result, the effective phase difference which is generatedby the combination of the liquid crystal composition layer and theoptically anisotropic medium is in the optimum range between 0.21 and0.36 μm.

In the twenty-seventh to twenty-nineth embodiments, the structure wasthe same as the third embodiment except as will be described. Thethickness of the liquid crystal composition layers were set to be as5.0, 5.2, and 5.5 μm, respectively. A nematic liquid crystal compositionhaving the refractive index anisotropy Δn of 0.072 (589 nm, 20° C.) wasused, so that the value for d·Δn was 0.360, 0.3744, and 0.396 μm,respectively. As the brightness and color tone are higher than thepreferable range between 0.21 to 0.36 μm, the liquid crystal cell iscolored orange. An optically anisotropic medium 28 (see FIG. 9) of anuniaxially stretched film made from polyvinyl alcohol was laminated overthe liquid crystal cell so as to compensate for the birefringent phasedifference of the liquid crystal at low voltage driving condition (inthese embodiments, zero volts). Therefore, φ_(R) was selected to be 85degrees, being the same as φ_(LC1) (=φ_(LC2)). The phase difference Rfwas 0.07, 0.08, and 0.10 μm, respectively, and the value for (d·Δn-Rf)was selected to be 0.29, 0.3044, and 0.296 μm, respectively, so as to bein the preferable range from 0.21 to 0.36 μm for the brightness and thecolor tone.

As the results, a bright display having the brightness exceeding 50%without coloring could be obtained.

Embodiment 30

In the thirtyth embodiment, the liquid crystal composition layer in thetwenty-seventh embodiment was replaced with a nematic liquid crystalcomposition (e.g. ZLI-4518 of Merck Co.) for which dielectric constantanisotropy Δ.di-elect cons. was negative with a value of -2.5, and itsΔn was 0.0712 (589 nm, 20° C.). The rest of the device was the same asthe twenty-first embodiment except as will be described below. Thethickness of the liquid crystal composition layers was set to be 5.5 μm.Thus, the value for d·Δn is 0.3916 μm. An optically anisotropic mediumof an uniaxially stretched film made from polyvinyl alcohol having aphase difference Rf of 0.11 μm was laminated over the liquid crystalcell so that the value for (d·Δn-Rf) became 0.2816 μm within thepreferable range from 0.21 to 0.36 μm for the brightness and the colortone.

As the results, a bright display having the brightness exceeding 50%without coloring could be obtained.

Embodiment 31

The structure of the thirty-first embodiment was generally the same asthe fifteenth embodiment except as will be described below.

In the thirty-first embodiment Δn of the liquid crystal compositionlayer was 0.072, and the gap was 7.0 μm. Therefore, d·Δn is 0.504 μm.φ_(LC1) was 89.5 degrees, the orientation direction of the liquidcrystal molecules at the upper interface and the lower interface wereset so as to cross over perpendicularly to each other, and the angle,|φ_(LC1) -φ_(LC2) |, was 90 degrees. The polarizing plates were arrangedperpendicularly to each other (|φ_(P2) -φ_(P1) |=90°), and therelationship with the orientation direction of the liquid crystalmolecules was selected so that φ_(LC1) =φ_(P1), so providing an opticalrotatory mode. As the result, a normally open type of liquid crystaldisplay device was obtained.

Measurement of the electro-optical characteristics in the thirty-firstembodiment revealed the result that the brightness exceeded 50%, therewas an extremely small difference in characteristics curve when thevisual angle was changed laterally and vertically, and scarcely changeddisplay characteristics could be obtained. The orientation of the liquidcrystal was satisfactory and no orientation failure domain wasgenerated.

Embodiments 32 and 33

The structures of the thirty-second and thirty-third embodiments weregenerally the same as the first embodiment except as will be describedbelow.

In these embodiment, the polarizing plates were so arranged that thedark state was obtained when an electric field of low level, not zero,was applied. The value of |φ_(LC1) -φ_(P1) | was set at 5 degrees in thethirty-second embodiment, and 15 degrees in the thirty-third embodiment,respectively, and the value of |φ_(P2) -φ_(P1) | was set at 90 degreesfor both thirty-second and thirty-third the embodiments.

A satisfactory display characteristic, for both brightness and visualangle was obtained, as in the other embodiments. Also, the liquidcrystal orientation was satisfactory, and no orientation failure domainwas generated.

Embodiments 34 and 35

The structures of the thirty-fourth and thirth-fifth embodiments weregenerally the same as the twenty-first embodiment except as will bedescribed below.

In the twenty-fourth and twenty-fifth embodiments, the polarizing plateswere arranged so that the dark state was obtained when an electric fieldof low level, not zero, was applied. The value of |φ_(P1) -φ_(LC1) | wasset at 5 degrees in the thirty-fourth embodiment and 7 degrees in thethirty-fifth embodiment, respectively, and the value of |φ_(P2) -φ_(P1)| was set as 90 degrees for both embodiments. The thickness d of theliquid crystal composition layer was 3 μm. Therefore, d·Δn was 0.275 μm.

The results of measurement of the electro-optical characteristicsmeasurement in these embodiments are shown in FIG. 13. For thethirty-fourth embodiment the voltage causing the dark state, V_(OFF),was 3.0 Volts and the voltage causing the maximum brightness, V_(ON),was 9.2 Volts. Therefore, sufficiently high contrast can be obtained ifits operation is performed with the voltage between V_(OFF) and V_(ON).Similarly, for the thirty-fifth embodiment, V_(OFF) was 5.0 Volts andV_(ON) was 9.0 Volts. When it was operated with a voltage between theV_(OFF) and the V_(ON), suitable display characteristics for bothbrightness and visual angle were obtained, as in the other embodiments.The liquid crystal orientation was satisfactory and no orientationfailure domain was generated.

Embodiment 36

The structure of the thirty-sixth embodiment was the same as thethirty-fourth embodiment except as will be described below.

In the thirty-sixth embodiment, image signals were supplied to thesignal electrode and at the same time, an alternating current at 3.0 Vwas applied to he common electrode. As a result, there was a reductionof the voltage which needed to be supplied to the signal electrode##EQU1## As its operation was performed with the voltage between V_(OFF)and V_(ON), a satisfactory display characteristics in both brightnessand visual angle was obtained, as in the other embodiments. The liquidcrystal orientation was satisfactory and no orientation failure domainwas generated.

Embodiment 37

The structure of the thirty-seventh embodiment was the same as the firstembodiment except as will be described below.

In the thirty-seventh embodiment, the polarizing plates were arranged sothat the dark state was obtained when an electric field of low level,not zero, was applied. The value of |φ_(LC1) -φ_(P1) | was set at 45degrees and the value of |φ_(P2) -φ_(P1) | was set at 90 degrees.Therefore, a bright state was obtained by applying a low voltage anddark state was obtained by applying a high voltage. The results ofmeasurement of the voltage dependence of the brightness in this presentembodiment is shown with a solid line in FIG. 14. Satisfactory displaycharacteristics for both brightness and visual angle were obtained, asin the other embodiments. The contrast ratio was 35. The, liquid crystalorientation was satisfactory, and no orientation failure domain wasgenerated.

Embodiment 38

With the structure of the thirty-seventh embodiment, a birefringentmedium (uniaxially stretched polyvinyl alcohol film) was insertedbetween the two polarizing plates for compensating interface residualphase difference. The stretched direction of the film φ_(R) was -45degrees, and the stretched direction crossed over perpendicularly to thetransmission axis of the polarized plate. The phase difference R_(f) was15 nm.

As shown by a dotted line in FIG. 14, the light leakage at high voltagewas suppressed more than in the thirty-seventh embodiment, and thecontrast ratio was improved to 150. In accordance with the presentinvention, it is possible to provide:

i) firstly, a thin film transistor type liquid crystal display devicehaving a high contrast without using a transparent electrode which canbe mass-produced with high yields at low cost by using inexpensivemanufacturing facilities;

ii) secondly, a thin film transistor type liquid crystal display devicehaving a satisfactory visual angle characteristics which can easilydisplay multiple-tone images;

iii) thirdly, a thin film transistor type liquid crystal display devicehaving a large margin for the processes of the liquid crystalorientation and materials, such a device can have a large aperturefactor, improved light transmission, and brighter images;

iv) fourthly, a thin film transistor type liquid crystal display deviceshaving a large aperture factor, improved light transmission, andbrighter images by providing simple structures for the thin filmtransistor structure.

These advantages may be achieved independently in combination dependingon the structure of the device.

What is claimed is:
 1. A liquid crystal display apparatus comprising:apair of substrates, and a liquid crystal layer interposed between saidpair of substrates, wherein:a substrate of said pair of substrates isprovided with a plural scanning signal electrodes; plural image signalelectrodes are arranged to intersect said plural scanning signalelectrodes in a matrix form; and a plurality of thin film transistorsare formed at locations corresponding to respective crossing points ofsaid plural image signal electrodes and said plural scanning signalelectrodes; at least a respective pixel is composed in respectiveregions surrounded by said plural scanning signal electrodes and saidplural image signal electrodes; wherein a respective pixel is providedwith a common electrode having plural portions extending in a samedirection as a direction of extension of said image signal electrodes,and at least a connecting portion extending over plural pixels in a samedirection as a direction of extension of said scanning signal electrodesand connected with said plural portions of said common electrodeextending in the same direction as the direction of extension of saidimage signal electrodes, and said respective pixel is provided with apixel electrode connected to an associated thin film transistor andextending in a same direction as the direction of extension of saidimage signal electrodes, said pixel electrode having at least a portionarranged between said plural portions of said common electrode; andwherein two portions among said plural portions of said common electrodeof adjacent pixels being adjacently arranged so that one of said imagesignal electrodes is interposed between said adjacent pixels.
 2. Aliquid crystal display apparatus as claimed in claim 1, wherein saidplural image signal electrodes, and said plural portions of said commonelectrodes arranged adjacently to said image signal electrodes areformed with an insulator interposed therebetween.
 3. A liquid crystaldisplay apparatus as claimed in claim 2, wherein said insulator isformed on said plural portions of said common electrode.
 4. A liquidcrystal display apparatus as claimed in claim 3, wherein said pluralimage signal electrodes are formed on said insulator.
 5. A liquidcrystal display apparatus as claimed in any one of claims 1 to 4,wherein said plural portions of said common electrodes and said pluralscanning signal electrodes are formed in a same layer.
 6. A liquidcrystal display apparatus as claimed in claim 5, wherein said insulatoris formed on said plural scanning electrodes.
 7. A liquid crystaldisplay apparatus as claimed in claim 1, wherein said at least a portionof said pixel electrode includes another portion overlapped with saidconnecting portion of said common electrode and having an insulatorinterposed therebetween, and a capacitance is formed at an overlappedportion of said another portion and said connecting portion.
 8. A liquidcrystal display apparatus as claimed in claim 7, wherein said insulatoris formed on said plural portions of said common electrode.
 9. A liquidcrystal display apparatus as claimed in any of claims 7 and 8, whereinsaid plural portions of said common electrode and said plural scanningsignal electrode are formed in a same layer, and said insulator isformed on said plural scanning signal electrode.
 10. A liquid crystaldisplay apparatus as claimed in claim 8, wherein said another portion ofsaid electrode is formed on said insulator.
 11. A liquid crystal displayapparatus as claimed in claim 9, wherein said another portion of saidelectrode is formed on said insulator.
 12. A liquid crystal displayapparatus as claimed in any of claims 1 to 4, 7, 8 and 10, wherein saidsurfaces of said plural portions of said common electrode are coatedwith at least one of a self-oxidized film and a self-nitridized film.13. A liquid crystal display apparatus as claimed in claim 5, whereinsaid surfaces of said plural portions of said common electrode arecoated with at least one of a self-oxidized film and a self-nitridizedfilm.
 14. A liquid crystal display apparatus as claimed in claim 6,wherein said surfaces of said plural portions of said common electrodeare coated with at least one of a self-oxidized film and aself-nitridized film.
 15. A liquid crystal display apparatus as claimedin claim 9, wherein said surfaces of said plural portions of said commonelectrode are coated with at least one of a self-oxidized film and aself-nitridized film.
 16. A liquid crystal display apparatus as claimedin claim 11, wherein said surfaces of said plural portions of saidcommon electrode are coated with at least one of a self-oxidized filmand a self-nitridized film.
 17. A liquid crystal display apparatus asclaimed in claim 1, wherein an electric field having a componentpredominantly parallel to one of substrates is generated in said liquidcrystal layer by applying a voltage between said common electrode andsaid pixel electrode.
 18. A liquid crystal display apparatus as claimedin claim 1, wherein said image signal electrode interposed between saidadjacent pixels is interposed between said two portions among saidplural portions of said common electrode of said adjacent pixels.
 19. Aliquid crystal display apparatus comprising:a pair of substrates, and aliquid crystal layer interposed between said pair of substrates,wherein:a substrate of said pair of substrates is provided with pluralscanning signal electrodes; plural image signal electrodes are arrangedto intersect said plural scanning signal electrodes in a matrix form;and a plurality of thin film transistors are formed at locationscorresponding to respective crossing points of said plural image signalelectrodes and said plural scanning signal electrodes; at least arespective pixel is composed in respective regions surrounded by saidplural scanning signal electrodes and said plural image signalelectrodes; wherein a respective pixel is provided with a commonelectrode having at least a portion extending in a same direction as adirection of extension of said image signal electrodes, and at least aconnecting portion extending over plural pixels in a same direction as adirection of extension of said scanning signal electrodes and connectedwith said at least a portion of said common electrode, and saidrespective pixel is provided with a pixel electrode connected to anassociated thin film transistor, said pixel electrode having at least aportion extending in a same direction as the direction of extension ofsaid image signal electrodes; wherein an insulator is interposed betweenat least one of said image signal electrodes and said at least a portionof said common electrode extending in the same direction as thedirection of extension of said image signal electrodes; and wherein saidat least a portion of said common electrode being arranged at a locationcloser to said at least one of said image signal electrodes than alocation of said at least a portion of said pixel electrode extending inthe same direction as the direction of extension of said image signalelectrodes to said at least one of said image signal electrodes.
 20. Aliquid crystal display apparatus as claimed in claim 19, wherein saidinsulator is formed on said at least a portion of said common electrode.21. A liquid crystal display apparatus as claimed in claim 20, whereinsaid plural image signal electrodes are formed on said insulator.
 22. Aliquid crystal display apparatus as claimed in claim 19, wherein anelectric field having a component predominantly parallel to said one ofsaid substrates is generated in said liquid crystal layer by applying avoltage between said common electrode and said pixel electrode.
 23. Aliquid crystal display apparatus as claimed in claim 19, wherein saidconnection portion of said common electrode extending in the samedirection as the direction of extension of said scanning signalelectrodes is disposed between a pair of adjacent scanning signalelectrodes.
 24. A liquid crystal display apparatus as claimed in claim23, wherein said connection portion of said common electrode extendingin the same direction as the direction of extension of said scanningsignal electrodes is disposed substantially in a central region betweensaid pair of adjacent scanning signal electrodes.
 25. A liquid crystaldisplay apparatus as claimed in claim 19, wherein said at least aportion of said common electrode is arranged at a location closer tosaid at least one of said image signal electrodes than a location ofsaid at least a portion of said common electrode to said at least aportion of said pixel electrode extending in the same direction as thedirection of extension of said image signal electrodes.
 26. A liquidcrystal display apparatus comprising:a pair of substrates, and a liquidcrystal layer interposed between said pair of substrates, wherein:asubstrate of said pair of substrates is provided with plural scanningsignal electrodes; plural image signal electrodes are arranged tointersect said plural scanning signal electrodes in a matrix form; and aplurality of thin film transistors are formed at locations correspondingto respective crossing points of said plural image signal electrodes andsaid plural scanning signal electrodes; at least a respective pixel iscomposed in respective regions surrounded by said plural scanning signalelectrodes and said plural image signal electrodes; wherein a respectivepixel is provided with a common electrode having at least a portionextending in a same direction as a direction of extension of saidscanning signal electrodes, and at least a portion extending in a samedirection as a direction of extension of said image signal electrodes,and said respective pixel is provided with a pixel electrode having atleast a portion extending in the same direction as the direction ofextension of a scanning signal electrode connected to an associated thinfilm transistor, and at least another portion extending in the samedirection as the direction of extension of said image signal electrodes;and wherein said at least a portion of said common electrode extendingin the same direction as said scanning signal electrode is overlappedwith said at least a portion of said pixel electrode extending in thesame direction as said scanning signal electrode with an insulatorinterposed therebetween.
 27. A liquid crystal display apparatus asclaimed in claim 26, wherein an electric field having a componentpredominantly parallel to one of substrates is generated in said liquidcrystal layer by applying a voltage between said common electrode andsaid pixel electrode.
 28. A liquid crystal display apparatus as claimedin claim 27, wherein said insulator is formed on said common electrode.29. A liquid crystal display apparatus as claimed in claim 28, whereinsaid pixel electrode is formed on said insulator.
 30. A liquid crystaldisplay apparatus as claimed in any one of claims 27 to 29, wherein thesurface of said common electrode is coated with at least one of aself-oxidized film and a self-nitridized film.
 31. A liquid crystaldisplay apparatus comprising:a pair of substrates, and a liquid crystallayer interposed between said pair of substrates, wherein:a substrate ofsaid pair of substrates is provided with plural scanning signalelectrodes; plural image signal electrodes are arranged to intersectsaid plural scanning signal electrodes in a matrix form; and a pluralityof thin film transistors are formed at locations corresponding torespective crossing points of said plural image signal electrodes andsaid plural scanning signal electrodes; at least a respective pixel iscomposed in respective regions surrounded by said plural scanning signalelectrodes and said plural image signal electrodes; wherein a respectivepixel is provided with a common electrode having at least a portionextending in a same direction as a direction of extension of said imagesignal electrodes, and connecting portions extending over plural pixelsin a same direction as a direction of extension of said scanning signalelectrodes and connected with said at least a portion of said commonelectrode, and said respective pixel is provided with a pixel electrodeconnected to an associated thin film transistor, and having at least afirst portion extending in the same direction as said image signalelectrodes, and a second portion connected to said first portion; andwherein said connecting portion of said common electrode and said secondportion of said pixel electrode is overlapped with an insulatorinterposed therebetween.
 32. A liquid crystal display apparatus asclaimed in claim 31, wherein an electric field having a componentpredominantly parallel to one of substrates is generated in said liquidcrystal layer by applying a voltage between said common electrode andsaid pixel electrode.
 33. A liquid crystal display apparatus as claimedin claim 32, wherein said insulator is formed on said common electrode.34. A liquid crystal display apparatus as claimed in claim 33, whereinsaid pixel electrode is formed on said insulator.
 35. A liquid crystaldisplay apparatus as claimed in any one of claims 31 to 34, wherein thesurface of said common electrode is coated with at least one of aself-oxidized film and a self-nitridized film.